Not difficult for true puzzle fiends. My son opened it within a quarter of an hour.
Sorry it wasn't enough of a challenge. Our puzzle boxes have an EU designer, but are made in China, hence the low price.
It is not something we sell, but you might like to look at the Revo Maze line of products - https://www.revohq.co.uk/ . They are designed and made in the UK, and the advanced ones quote a 'solving time' of 100 - 200 hours! They are not cheap, but maybe if you look at the cost of each 'puzzling hour' it is not so bad! Having said that, I would probably not start with one of the advanced models...!